最小 MCP サーバー

pip install mcp   # 公式 Python SDK

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("demo-server")

@mcp.tool()
def calculator(expression: str) -> str:
    """数式を評価して結果を返す電卓ツール。

    Args:
        expression: Python で評価可能な数式。例: '2 * (3 + 4)'
    """
    import re
    if not re.match(r"^[\d\s\+\-\*\/\(\)\.\%]+$", expression):
        return f"Error: 不正な式 {expression!r}"
    try:
        return str(eval(expression, {"__builtins__": {}}, {}))
    except Exception as e:
        return f"Error: {e}"


@mcp.tool()
def echo(message: str) -> str:
    """受け取ったメッセージをそのまま返すデバッグ用ツール。"""
    return f"echo: {message}"


if __name__ == "__main__":
    mcp.run()  # stdio transport で起動

@mcp.resource("handbook://chapter/{chapter_num}")
def read_chapter(chapter_num: int) -> str:
    """指定章のハンドブック本文を返す。"""
    return HANDBOOK_CHAPTERS[chapter_num]


@mcp.prompt()
def review_prompt(code: str) -> str:
    """コードレビュー用のプロンプトテンプレート。"""
    return f"以下のコードをレビューしてください:\n\n```\n{code}\n```"

python server.py
# → stdio 経由で待機開始

mcp dev server.py  # 開発モード、ブラウザ UI で inspector

npx @modelcontextprotocol/inspector python server.py

from mcp.client.stdio import stdio_client, StdioServerParameters
from mcp import ClientSession

async def main():
    params = StdioServerParameters(command="python", args=["server.py"])
    async with stdio_client(params) as (read, write):
        async with ClientSession(read, write) as session:
            await session.initialize()
            tools = await session.list_tools()
            result = await session.call_tool("calculator", {"expression": "2+3"})
            print(result.content[0].text)  # "5"

import logging
logging.basicConfig(level=logging.DEBUG)

# stdio で手動操作(テスト用)
echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1"}}}' | python server.py